Comparative Study of Dynamic Stall under Pitch Oscillation and Oscillating Freestream on Wind Turbine Airfoil and Blade

Abstract

This study aims to assess the dynamic stall of the wind turbine blade undergoing pitch oscillation (PO) and oscillating freestream (OF), respectively. Firstly, a thin-airfoil theoretical analysis was performed to differentiate between these two dynamic effects. During upstroke, PO results in a positive effective airfoil camber, while OF has an additional negative effective airfoil camber, and yet in contrast during downstroke, PO decreases the effective camber, while OF increases the effective camber. Secondly, the equivalence relation between PO and OF is investigated by numerically solving the unsteady Reynolds-averaged Navier-Stokes equations. The difference between PO and OF mainly exists in the linear part of the aerodynamic loads. Because the difference is great at high reduced frequencies or angle of attack (AOA) amplitudes, PO and OF should be treated separately for dynamic stall from different aerodynamic sources. Thirdly, the Beddoes-Leishman dynamic model coupled with Bak's rotational stall delay model was used to predict the yawed responses of the blade section. The obtained results show different aerodynamic responses between PO and OF, although consideration of rotational augmentation can greatly improve the accuracy of the lift and drag coefficients. To improve the understanding and coupling modeling of rotational augmentation and dynamic stall, an extended analysis of the coupled effect was performed as well.

Combined Effect of Rotational Augmentation and Dynamic Stall on a Horizontal Axis Wind Turbine

“…To predict the dynamic stall of airfoil, we use the commercial solver ANSYS/FLUENT 16.0 to solve the unsteady Reynolds-averaged Navier-Stokes equations [32]. An equivalent planar motion of the rotating blade section is obtained by varying incident velocity in the magnitude and direction, because the unsteady effects under yawed inflow conditions results from the induction of varying flow field [3,33].…”

“…As described in our previous work [33,34], the mesh around the airfoil geometry is generated in a structured O-type configuration to assure the wall orthogonality. The first layer spacing is 10 −5 c to assure a good resolution of the viscous sublayer, with the growth rate being 1.08.…”

Numerical Investigation of Passive Vortex Generators on a Wind Turbine Airfoil Undergoing Pitch Oscillations

“…In order to obtain a good resolution, the third-order MUSCL convection scheme [25] is used for spatial discretization of the whole set of URANS and turbulence equations, the bounded second-order implicit scheme [25] for time differencing, and the pressure-based Coupled algorithm [25] for the pressure-velocity coupling. Based on experience with previous computations [26], the time step is selected to assure 540 steps computed over each cycle with 20 inner iterations per time step. The turbulence is modelled by the SST k-ω eddy viscosity model [27] incorporated with the γ-Re θ transition model [28], because the dynamic-stall predictions can be enhanced by considering the transitional flow effect [21].…”

“…The computational mesh is generated in the same manner, with about 240 wrap-around points and 200 normal layers. The mesh independency study has been carried out in our previous work [26].…”

Numerical Investigation of Jet Angle Effect on Airfoil Stall Control

“…In contrast to the rotary wing, fixed-wing aircraft can use dynamic stall to improve performance such as super-maneuverability [8][9][10]. Wind turbine blades also experience dynamic stall under highly unsteady conditions [11][12][13][14].…”